In the generic designation of polyethylene, there are involved commonly used polyethylene (general-purpose polyethylene) of a molecular weight of 50,000-200,000 or thereabouts, low molecular weight polyethylene (so-called wax), high-molecular weight polyethylene and ultra-high-molecular weight polyethylene.
Of such polyethylene as referred to above, the ultra-high-molecular weight polyethylene generally has a molecular weight of more than 1,000,000 and possesses characteristics entirely different from those of general-purpose polyethylene. That is, because of extremely excellent characteristics such as impact resistance, abrasion resistance, chemical resistance and tensile strength, this ultra-high-molecular weight polyethylene has come to be newly used as engineering plastics, taking advantage of such characteristics as mentioned above.
The ultra-high-molecular weight polyethylene possesses such excellent characteristics on the one hand, but, on the other hand, has an extremely high intrinsic viscosity because of its high molecular weight. Accordingly, this ultra-high-molecular weight polyethylene is extremely low in flowability. In preparing molded articles from the ultra-high-molecular weight polyethylene, therefore, there is scarcely employed the extrusion molding or injection molding method used so far in the preparation of molded articles from general-purpose polyethylenes.
Where desired molded articles are prepared using such ultra-high-molecular weight polyethylene as illustrated above, there is adopted a method which comprises preparing in advance a mixture by adding a small amount of the ultra-high-molecular weight polyethylene to a diluent, and forming the resulting mixture into an article of a desired shape, followed by removing the diluent therefrom.
For example, Japanese Patent L-O-P No. 81612/1983 discloses a process for preparing filaments having a tensile strength of more than 1.5 GPa by using a solution containing not more than 20% by weight of an ethylene polymer or copolymer having a weight average molecular weight of more than 400,000, wherein the weight average molecular weight/number average molecular weight ratio (Mw/Mn) is less than 5, and at least 80% by weight of a solvent.
In the polyethylene solution used in this process, however, the upper limit of the polyethylene concentration as defined is 20% by weight, because the polyethylene used is high in molecular weight. Concretely, even such polyethylene as having a weight average molecular weight of 500,000 is used for preparing a solution of polyethylene, the polyethylene concentration of the resulting solution used is defined as low as 8% by weight. And, the filaments obtained by the use of such polyethylene solution as mentioned above come to have a tensile strength of about 1.9 GPa (Mw/Mn=2.9). Where filaments having a tensile strength of more than 2 GPa, the polyethylene used therefore must have a very high molecular weight such as about 1,100,000, and the polyethylene concentration of a solution containing such polyethylene as mentioned above is merely about 2% by weight.
Japanese Patent L-O-P Publn. No. 167010/1986 discloses a process for preparing polyethylene molded articles having a tenacity of at least 13 g/denier and a modulus of 350 g/denier by the use of a solution preparing by mixing polyethylene having a weight average molecular weight of 200,000-4,000,000 with a diluent. Basically, this process is technically identical with the process disclosed in the above-mentioned Japanese Patent L-O-P Publn. No. 81612/1983, and the polyethylene concentration of the polyethylene solution actually used is merely about 10% by weight.
In this manner, when molded articles are prepared from ultra-high-molecular weight polyethylenes, there must be used large amounts of the diluent, and it is difficult to improve markedly the working efficiency in the process for preparing the molded articles.
As stated above, when polyethylene fiber having an excellent tensile strength such as more than 1.5 GPa is intended to be prepared, the ultra-high-molecular weight polyethylene so far used therefore had a molecular weight of more than about 600,000, preferably more than 1,000,000. The reason therefor is that, because deterioration in tensile characteristics, especially tensile strength, has been considered to be attributable to structural defect in the terminal of molecule of the resulting fiber, the higher is the molecular weight of polyethylene, the higher is the strength of the resulting fiber. For example, Japanese Patent L-O-P Publn. No. 66316/1988 discloses a process for obtaining filaments excellent in tensile characteristics, which comprises preparing gel filaments from a dilute solution of ultra-high-molecular weight polyethylene having a molecular weight of more than 600,000, and stretching the resulting gel filaments. In this publication, actually used is the ultra-high-molecular weight polyethylene having a molecular weight reaching even about 1,500,000.
Japanese Patent L-O-P Publn. No. 187614/1984 also discloses a process for preparing polyethylene fiber, and according to the example of the process, the polyethylene fiber having a fiber diameter of 4.2 deniers and a strength of 3.16 GPa is obtained.
Furthermore, Japanese Patent L-O-P Publn. No. 240432/1985 discloses a process for preparing stretched articles excellent in tensile characteristics, which comprises melt kneading a mixture of ultra-high-molecular weight polyethylene having an intrinsic viscosity of more than 5 dl/g and an aliphatic hydrocarbon derivative, and then extruding the resulting kneadate through a die, followed by stretching. Concretely, it is reported according to this publication that the polyethylene fiber having a maximum tensile strength of 3.04 GPa is obtained from the ultra-high-molecular weight having an intrinsic viscosity of 8.2 dl/g.
However, the higher is the molecular weight of polyethylene, the poorer is the moldability of the polyethylene. For example, where filaments excellent in tensile characteristics are intended to be prepared, when the molecular weight of the polyethylene increases higher and higher, and the solubility in solvent of the polyethylene decreases or the viscosity of the polyethylene solution as a given concentration becomes exceedingly high, with the result that the resulting filaments tend to break in the spinning or stretch step. Accordingly, when polyethylene having a very high molecular weight is used, it was necessary to lower the concentration of the polyethylene solution or slow down the spinning speed or stretching speed in order to avoid occurrence of such troubles as mentioned above. In this manner, fiber having excellent tensile characteristics can be obtained when the starting polyethylene used is higher in molecular weight, but on the other hand, because of poor moldability of the polyethylene having a higher molecular weight, a decrease in industrial productivity of the resulting fiber was unavoidable.
The ultra-high-molecular weight polyethylene having a very high molecular weight are liable to heat degradation at the time of molding thereof. In that account, a decrease in molecular weight to a certain degree is unavoidable at the time of preparing this starting polyethylene or at the time of spinning the same. Furthermore, it is considered that carbonyl group or the like formed at the time of heat degradation of the starting polyethylene will have an adverse effect on weathering properties of the resulting polyethylene fiber.
In this connection, polyethylene fiber high in tenacity is used mainly for making woven fabrics. Such woven fabrics are used concretely as reinforced textile fabrics, bulletproof fabrics and cut-protective clothes. For application of such polyethylene fiber is composite material reinforcing fiber, the advent of high tenacity polyethylene fiber smaller in fineness is desired from the standpoint of weave density and adhesion properties of the fiber at the time of weaving thereof, and for application of such polyethylene fiber in bulletproof fabrics and cut-protective clothes, the advent of polyethylene fiber excellent in tenacity and small in fineness is desired from the standpoint of bulletproofing properties and resistance to cutting and the advent of the polyethylene fiber excellent in touch and small in fineness is desired from the standpoint of a bodily sensation.
Molecular orientation articles molded from high molecular weight polyethylene are sometimes used as composite material reinforcing fiber in which excellent adhesion between the fiber and matrix is required. However, polyethylene is generally not so excellent in adhesion properties, and hence there are adopted various measures to improve the polyethylene in adhesion properties, and known as one of such measures is corona discharge treatment.
When molecular orientation articles molded from high molecular weight polyethylenes are subjected to corona discharge treatment, the thus treated articles sometimes deteriorated greatly in tensile strength, though they improved in adhesion properties.
The present inventors conducted extensive researches with the view of overcoming such drawbacks associated with high molecular weight polyethylenes as mentioned above, and eventually have accomplished the present invention on the basis of their finding that even from high molecular weight polyethylene having a weight average molecular weight of not more than 600,000 and excellent in moldability, there is obtained fine polyethylene fiber, i.e., a molecular orientation article formed from polyethylene, which hardly deteriorates in tensile strength and is excellent in tenacity even when said article is subjected to corona discharge treatment, by making finer said fine polyethylene fiber when it is formed from the high molecular weight polyethylene.